Disc drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disc drive comprises a magnetic disc that is rotated by a spindle motor. The surface of the disc is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter.
Each of the data tracks extends generally circumferentially around the disc and can store data in the form of magnetic transitions within the radial extent of the track on the disc surface. An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data, or to transmit an electric signal that causes a magnetic transition on the disc surface, to write data. The magnetic transducer includes a read/write gap that contains the active elements of the transducer at a position suitable for interaction with the magnetic surface of the disc. The radial dimension of the gap fits within the radial extent of the data track containing the transitions so that only transitions of the single track are transduced by the interactive element when the interactive element is properly centered over the respective data track.
The magnetic transducer is mounted by a head structure to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disc to either read data from or write data to the preselected data track of the disc, as the disc rotates below the transducer. The actuator arm is, in turn, mounted to a voice coil motor that can be controlled to move the actuator arm across the disc surface.
A servo system is typically used to control the position of the actuator arm to insure that the head is properly centered over the magnetic transitions during either a read or write operation. In a known servo system, servo position information is recorded on the disc surface between written data blocks, and periodically read by the head for use in a closed loop control of the voice coil motor to position the actuator arm. Such a servo arrangement is referred to as an embedded servo system.
In modern disc drive architectures utilizing an embedded servo, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector. Associated with the data sectors are a series of servo sectors, generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors.
Each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine head position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to, e.g., a microprocessor within the disc drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
Each servo sector also contains encoded information to uniquely identify the specific track location of the head. For example, each track can be assigned a unique number, which is encoded using a Gray code and recorded in each servo sector of the track. The Gray code information is used in conjunction with the servo bursts to control movement of the actuator arm when the arm is moving the head in a seek operation from a current track to a destination track containing a data field to be read or written.
The head structure also includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disc surface due to fluid currents caused by rotation of the disc. Thus, the transducer does not physically contact the disc surface during normal operation of the disc drive to minimize wear at both the head and disc surface. The amount of distance that the transducer flies above the disc surface is referred to as the “fly height.” By maintaining the fly height of the head at an even level regardless of the radial position of the head, it is ensured that the interaction of the head and magnetic charge stored on the media will be consistent across the disc.
When writing or reading information, the hard disc drive may perform a seek routine to move the transducers from one cylinder (track) to another cylinder. During the seek routine the voice coil motor is excited with a current to move the transducers to the new cylinder location on the disc surfaces. The controller also performs a servo routine to insure that the transducer moves to the correct cylinder location, and is at the center of the track.
Many disc drives utilize a “model reference” control algorithm to move the transducer to the correct location in the shortest amount of time. An example of the waveform for seek routines that utilize model reference control theory is provided in FIG. 1. As shown in FIG. 1, the waveform has an initial acceleration phase, illustrated as being the positive square wave portion between t0 and ta, with ta being the time at the end of the acceleration state. During this phase, a constant acceleration current is applied to the voice coil motor to cause it to accelerate the assembly.
Following the initial acceleration phase, there is a coast phase in which there is no current applied, illustrated in FIG. 1 as the period between ta and tc, where tc is the time at the end of the coast state. During this phase of operation, the voice coil motor does not generate any further acceleration of the assembly and it is allowed to coast as part of the seek operation. Following the coast phase, there is a deceleration phase illustrated as the period between tc and tf, where tf is the final time of the seek operation. During this phase of operation, a deceleration current is applied to the voice coil motor which causes the assembly to reduce its speed down to zero.
Because these conventional model reference seek models include the coast phase, such seeks typically require more time than would otherwise be necessary if the coast phase were able to be removed. However, there is no known approach that provides a feasible way to remove the coast phase of the model reference seek without introducing undesirable affects including high frequency harmonics which stimulate mechanical resonance and excite mechanical components or assemblies with high natural frequencies.
In order to reduce the amount of time for a seek operation, a “bang-bang” control algorithm may be utilized to move the transducer to the correct location in the shortest amount of time. As discussed in U.S. Pat. No. 6,549,364, for example, the shape of such a waveform generated using a “bang-bang” control algorithm is a square waveform. Unfortunately, square waveforms contain high frequency harmonics which stimulate mechanical resonance in the hard drive assembly and excited mechanical components which high natural frequencies. This results in acoustic noise, undesirable vibration, and associated settling time due to residual vibration. The mechanical resonance created by the square waveforms of tend to increase both the settling and overall time required to write data or read information from the disc.
The present invention provides a solution to this and other problems, and offers other advantages over previous solutions.